This invention relates to a method of and apparatus for the inspection of fluids. In particular, the invention relates to an improved apparatus for measuring the color properties of fluids in transmission and/or reflection, such as paint dispersions and tints flowing through the apparatus.
Pigment dispersions and tints are widely used nowadays in formulating high performance coating compositions used in particular for exterior finishes for automobiles and trucks.
In the manufacture of such dispersions and tints, one problem is to measure the color and strength of the material as it is being made, so that adjustments can be quickly made to bring this material within acceptable color tolerance values. Color measurements nowadays are carried out by a manual process, which involves taking an aliquot of the material, blending it with a standard white or black paint, spraying out the blends as a coating onto panels, baking and drying the panels, and then measuring one or more color properties of the dried coating using a colorimeter or spectrophotometer against a standard. Adjustments are then made to the batch until the color parameters match those of the standard.
Color measurements by this method are very time consuming because of sample preparation and drying times. Also, this procedure may have to be repeated numerous times before the desired color property is achieved. Another problem which arises with this procedure is that the accuracy of the test is dependent on the color and strength stability of the standard white or black paints. Even with careful control, these standards tend to vary from batch to batch and also tend to flocculate or settle in time, leading to poor test repeatability and making it very difficult to accurately analyze the color and strength of the batch as it is being made.
The aim within the industry for some time has been to measure the color properties of these fluids in a wet state and in a way which predicts the color of the fluid when applied and dried. The primary benefits are mainly associated with time savings although some are associated with the increased likelihood of an automated manufacturing process.
For measuring in transmission, conventional laboratory spectrophotometers, employing cuvette-type sample chambers, have been proposed to make such wet measurements off-line by measuring a transmission spectrum of a wet transparent sample. However, cell pathlengths in such spectrophotometers are, in general, too large for such measurements, as these fluids tend to be too optically dense. Moreover, settling and flocculation can also occur, changing the color of the sample. Additionally, simply taking a sample of wet fluid and putting it in a glass cell and measuring its color properties generally leads to inconsistent results, mostly due to poor repeatability and operator variability.
For measuring in reflection mode, one might propose using a conventional colorimeter to measure the free surface of a wet coating of fluids such as pigment dispersions or tints. However, surface non-uniformities of such coatings, as well as settling, flocculation, and lack of hiding would still lead to erroneous results and unacceptable measurement variability. Moreover, coupling such a device to a wet sample has its own difficulties, including but not limited to, operation of said device in the presence of volatile flammable solvents emitted from the sample surface.
Another instrument, described in Batista et al. WO 98/16822, published Apr. 23, 1998, employing a variable pathlength fluid measurement cell to measure properties of fluids, including color, could be used for such measurements. However, this equipment possesses multiple moving parts which are part of the fluid path, which can cause difficulty in cleaning, and are difficult to maintain. Another disadvantage is that the design is such that it requires high volumes of fluid sample to take proper readings.
Therefore, there is still a need to provide a method and apparatus for color measurement of wet fluids that: produces acceptably consistent results; does not require the spraying and blending with white or black standards and the production of a number of dry samples; cleans rapidly (within 1 or 2 minutes) so that the cycle time of the measurement is extremely small compared to process changes; provides an easy means (including automatic) of delivering sample to the analysis cell so that fluid measurements of color and strength can be made rapidly; and predicts with confidence that the wet readings will also match the standard in the dry.
In addition to the above features, there is also a need to provide a method and apparatus that can be made intrinsically safe, so that it can be placed on a plant floor in an environment wherein may be contained an explosive atmosphere